It sounds like a well worn narrative: A Tea Party-backed candidate is giving a long-serving establishment candidate a run for his money. But the Texas Republican Senate primary race to fill the seat left open by Kay Bailey Hutchison's retirement is not your typical moderate vs. far right candidate match-up.

The Republican mainstream candidate is Lt. Gov. David Dewhurst, deputy to Gov. Rick Perry, who is a Tea Party favorite and widely considered to be one of the country's most conservative governors. Dewhurst and Ted Cruz, a former solicitor general, are both touting their conservative credentials, and both of them have evidence to back up their claims.

Polling indicates that next Tuesday's contest has come down to a race between Dewhurst and Cruz. Former Dallas Mayor Tom Leppert and former ESPN analyst Craig James are also running but lag behind Dewhurst and Cruz in the polls.

The establishment and Tea Party labels have been flaunted by Cruz, who brands himself the "anti-establishment contestant" in the race.

"This race is ground zero in the wave between the moderate establishment and the Tea Party movement that swept the ground in 2010," Cruz told ABC News.

Cruz, 41, certainly checks off the Tea Party boxes. An avid proponent of slashing spending and reducing the deficit, Cruz has been endorsed by major Tea Party stars such as Sarah Palin, Jim DeMint and Pat Toomey, and he's received financial backing from the Club for Growth, a conservative PAC that focuses on limited government. The club has spent roughly $2 million on Cruz.

But Dewhurst, 66, also packs strong conservative credentials. Perry has endorsed the lieutenant governor, as has Mike Huckabee -- both have even appeared in ads on Dewhurst's behalf.

"I have the most fiscally conservative and socially conservative record of any lieutenant governor in the history of Texas," Dewhurst told ABC News. "It's impossible to call me anything but a strong conservative."

Both candidates come with impressive resumes. Cruz, the son of a Cuban immigrant, is frequently compared to Sen. Marco Rubio, R-Fla. He was the first Latino solicitor general in Texas history, and if wins the race, he would become the first Latino senator from Texas. He graduated from Princeton University and Harvard Law School, where he was editor of the Law Review, and he clerked for the late Supreme Court Justice William Rehnquist.

Dewhurst has had a long career in public service, and spent many years in the private sector. He has served as lieutenant governor since 2002, and was previously the Texas land commissioner. A former Air Force officer who worked for the CIA, Dewhurst helped to establish Falcon Seaboard, an energy investment company based in Houston. Dewhurst is worth an estimated $200 million.

Even though the primary is not until May 29, more than 200,000 ballots have already been cast in the Republican primary through early voting, according to the Texas secretary of state.

Not that the candidate will not necessarily be known the next day. Texas' election code requires that a candidate receive more than 50 percent of the vote to win the nomination outright and avoid a runoff. If no candidate reaches the 50 percent threshold, the top two finishers face off again July 31.

Recent polling shows Dewhurst leading the field, but falling short of the 50 percent mark, with Cruz in a strong second place. So a Dewhurst-Cruz runoff is a strong possibility. It's that scenario that Cruz is angling for, as it doesn't appear likely that he could come up from behind to claim 50 percent in the primary.

"If we get to a runoff, we win decisively," Cruz said. "In a runoff, we will have a decisive advantage. It will be on July 31, the dead of summer, turnout will drop significantly. The only voters who will show up in a runoff are the hardcore, committed primary voters. Polling shows we have a double-digit lead today among informed primary voters."

Dewhurst, on the other hand, said, "My strategy has always been, in this race and in previous races, to be the top vote-getter, and as long as I do that then we'll either win the primary on Tuesday night or we'll win the primary on the runoff date on July 31."

While the political implications of a runoff are debated, there's one aspect of the race that seems clear: The money will continue to flow into Texas until the party has a nominee.

"I think it's inevitable that spending will increase if there's a runoff," said Cruz.

Spending has already exceeded $25 million, according to the Center for Responsive Politics, making it the most expensive U.S. Senate race so far this cycle.

The sheer geography of the Lone Star State makes it an extremely expensive place to campaign. Texas is simply too big for a candidate to feasibly reach every part of it without taking to the airwaves, and Texas media markets are costly.

Both sides report raising around $6 million. Dewhurst has sunk a portion of his own fortune into the race, too, spending a total of $8 million of his own money in his Senate bid, according to campaign disclosures.

The candidate who ultimately wins the primary in this solidly red state is widely expected to win the Senate race in the fall.